1. Field of the Invention
The present invention relates to a liquid crystal display device to be used for television and other display apparatuses, to a method of fabricating the same and, more particularly, to a liquid crystal display device that uses a liquid crystal material containing a photosensitive material and a method of fabricating the same.
2. Description of the Related Art
A liquid crystal display device is a display device that comprises a liquid crystal sealed between two opposing substrates and that uses electrical stimulus for optical switching by exploiting the electro-optical anisotropy of a liquid crystal. Utilizing the refractive index anisotropy that the liquid crystal possesses, the brightness of the light transmitted by the liquid crystal panel is controlled by applying a voltage to the liquid crystal and thereby reorienting the axis of the refractive index anisotropy.
In such a liquid crystal display device, it is extremely important to control the alignment of liquid crystal molecules when no voltage is applied to the liquid crystal. If the initial alignment is not stable, when a voltage is applied to the liquid crystal, the liquid crystal molecules do not align in a predictable manner, resulting in an inability to control the refractive index. Various techniques have been developed to control the alignment of liquid crystal molecules, representative examples including a technique that controls the initially formed angle (pretilt angle) between the alignment film and the liquid crystal and a technique that controls the horizontal electric field formed between the bus line and the pixel electrode.
The same can be said of a display device that uses a liquid crystal material containing a photosensitive material; specifically, in a liquid crystal display mode in which the initial alignment is controlled by radiation of light in the presence of an applied voltage, the voltage application method during the radiation becomes important. The reason is that, if the magnitude of the applied voltage differs, a change will occur in the initially formed pretilt angle, resulting in a change in transmittance characteristics.
In connection with a first aspect of the invention, techniques called passive matrix driving and active matrix driving have usually been used to drive liquid crystals; nowadays, with an increasing demand for higher resolution, the active matrix display mode that uses thin-film transistors (TFTs) is the dominant liquid crystal display mode. In a liquid crystal display having such TFTs, when radiating light onto the liquid crystal while applying a voltage to it, it is usually practiced to expose the liquid crystal to light radiation while applying a TFT ON voltage to each gate bus line and a desired voltage to each data bus line, as shown in FIGS. 1 and 2.
However, when such a liquid crystal exposure method is employed, if there is a line defect due to a bus line break or short, as shown in FIG. 3, the liquid crystal will be exposed to light when the liquid crystal in the affected area cannot be driven, and a pretilt angle different from that in other areas will be formed in this defect area, resulting in the problem that the brightness in this area differs from the brightness in other areas.
Or, in the TFT channel ON state, a shift in the TFT threshold value can occur due to exposure to ultraviolet radiation, as shown in FIG. 4, resulting in the problem that the region where the TFTs can be driven stably shifts from the desired region.
On the other hand, in connection with a second aspect of the invention, displays using the TN mode have been the predominant type of active matrix liquid crystal display, but this type of display has had the shortcoming that the viewing angle is narrow. Nowadays, a technique called the MVA mode or a technique called the IPS mode is employed to achieve a wide viewing angle liquid crystal panel.
In the IPS mode, liquid crystal molecules are switched in the horizontal plane by using comb-shaped electrodes, but a strong backlight is required because the comb-shaped electrodes significantly reduce the numerical aperture. In the MVA mode, liquid crystal molecules are aligned vertically to the substrates, and the alignment of the liquid crystal molecules is controlled by the use of protrusions or slits formed in a transparent electrode (for example, an ITO electrode). The decrease in the effective numerical aperture due to the protrusions or slits used in MVA is not so large as that caused by the comb-electrodes in IPS, but compared with TN mode displays, the light transmittance of the liquid crystal panel is low, and it has not been possible to employ MVA for notebook computers that require low power consumption.
When fine slits are formed in the ITO electrode, the liquid crystal molecules tilt parallel to the fine slits, but in two different directions. If the fine slits are sufficiently long, liquid crystal molecules located farther from a structure such as a bank that defines the direction in which the liquid crystal molecules tilt are caused to tilt randomly in two directions upon application of a voltage. However, the liquid crystal molecules located at the boundary between the liquid crystal molecules caused to tilt in different directions, cannot tilt in either direction, resulting in the formation of a dark area such as that shown in FIG. 29. Further, in a structure where the liquid crystal molecules are caused to tilt in two different directions in order to improve viewing angle, if there are liquid crystal molecules that are caused to tilt in the opposite direction, as shown in FIG. 29, the viewing angle characteristics degrade.
In connection with a third aspect of the invention, in an LCD (MVA-LCD) in which an N-type liquid crystal is aligned vertically and in which, upon application of a voltage, the molecules of the liquid crystal are caused to tilt in a number of predefined directions by using alignment protrusions or electrode slits, the liquid crystal molecules are almost completely vertically aligned in the absence of an applied voltage, but are caused to tilt in the various predefined directions when a voltage is applied. The tilt directions of the liquid crystal molecules are controlled so that they always make an angle of 45° to the polarizer absorption axis, but the liquid crystal molecules as a continuum can tilt in a direction intermediate between them. Furthermore, areas where the tilt direction of the liquid crystal molecules is displaced from the predefined direction inevitably exist because of the effects of the horizontal electric field, etc. at the time of driving or irregularities in the structure. In normally black displays where the polarizers are arranged in a crossed Nicol configuration, this means that dark areas appear when the display is driven in the white display state, and the screen brightness thus decreases. To address this problem, in a liquid crystal display device constructed by sandwiching between two substrates a liquid crystal composition containing a photopolymerizable or thermally polymerizable component, there is employed a technique that polymerizes the polymerizable component while applying a voltage, thereby defining the direction in which the liquid crystal molecules tilt in the presence of an applied voltage.
With this technique, however, if the polymerization is insufficient, image sticking can occur. This is believed to occur because the rigidity of the polymerized polymer is insufficient and deformation occurs due to the realignment of the liquid crystal molecules at the time of voltage application. On the other hand, to sufficiently polymerize the polymer, the duration of light radiation must be increased, but in that case, take time at the time of volume production becomes a problem.
In connection with a fourth aspect of the invention, conventional liquid crystal display devices predominantly use the TN mode in which horizontally aligned liquid crystal molecules are twisted between the top and bottom substrates, but gray-scale inversion occurs in the mid gray-scale range because the tilt angle of the liquid crystal differs depending on the viewing direction, that is, the viewing angle. To address this, a technique called the MVA mode has been proposed in which vertically aligned liquid crystal molecules are tilted symmetrically in opposite directions to compensate for the viewing angle. In this technique, alignment control members made of an insulating material are formed on electrodes to control the liquid crystal tilt directions. However, since the liquid crystal molecules tilt in 180° opposite directions on both sides of each alignment control member, a dark line is formed and transmittance decreases. To obtain sufficient transmittance, it is preferable to reduce the area occupied by the alignment control members by forming them spaced farther apart, but this would in turn slow the propagation speed of the tilt, resulting in a slow response speed.
To address this, a technique has been proposed in which a liquid crystal composition containing a polymerizable component is sandwiched between substrates and, while applying a voltage, the polymerizable component is polymerized, thereby defining the tilt direction of the liquid crystal molecules. This achieves a faster response speed while retaining the transmittance.
However, in the case of a liquid crystal display device in which the tilt direction of the liquid crystal molecules is defined by polymerizing the polymerizable component in the liquid crystal while applying a voltage, there arises the problem that display unevenness occurs after the polymerization of the polymerizable component, because of the separation of the liquid crystal and the polymerizable component which occurs when the liquid crystal material is injected at high speed at the initial stage of injection or when there is an abrupt change in speed near a frame edge.
In connection with a fifth aspect of the invention, in a liquid crystal display device, it has traditionally been practiced to control the alignment direction of the vertically aligned panel by a TFT substrate having slits in pixel electrodes and a color filter substrate having insulating protrusions, and it has therefore been necessary to form the dielectric protrusions on one of the substrates. Fabrication of such a liquid crystal display device therefore has involved the problem that the number of processing steps increases.
Furthermore, forming the protrusions within display pixels leads to the problem that the numerical aperture decreases, reducing the transmittance. In view of this, it has been proposed to control the alignment of the liquid crystal molecules by a polymerizable component added in the liquid crystal, in order to achieve multi-domains without using dielectric layer protrusions. That is, the liquid crystal to which the polymerizable component is added is injected into the panel and, while applying a voltage, the polymerizable component is polymerized, thereby controlling the alignment of the liquid crystal molecules.
However, if the polymer composition that defines the alignment direction does not have a sufficient cross-linked structure, the polymer becomes flexible, and its restoring force weakens. If the polymer has such properties, then, when a voltage is applied to the liquid crystal to cause the liquid crystal molecules to tilt, and the liquid crystal is still held in that state, the pretilt angle of the liquid crystal does not return to its initial state even after the applied voltage is removed. This means that the voltage-transmittance characteristic has changed, and this defect manifests itself as a pattern image sticking.
In connection with a sixth aspect of the invention, in an MVA-LCD in which liquid crystals having a negative dielectric anisotropy are vertically aligned, and in which the alignment of the liquid crystal in the presence of an applied voltage is controlled in a number of predefined directions, without using a rubbing treatment but by utilizing the banks or slits formed on the substrates, the LCD provides excellent viewing angle characteristics compared with conventional TN mode LCDs, but there is a disadvantage that white brightness is low and the display is therefore relatively dark. The major reason is that portions above the banks or slits correspond to the boundaries across which the liquid crystal alignment changes, and these portions appear optically dark, reducing the transmittance of white. To improve this, the spacing between the banks or slits should be made sufficiently wide, but in that case, as the number of banks or slits for controlling the liquid crystal alignment decreases, it takes time until the alignment stabilizes, thus slowing the response speed.
To obtain a brighter, faster response MVA panel by alleviating the above deficiency, it is effective to use a technique in which a liquid crystal composition containing a polymerizable component is sandwiched between substrates and, while applying a voltage, the polymerizable component is polymerized, thereby defining the tilt direction of the liquid crystal molecules. For the polymerizable component, a monomer material that polymerizes by ultraviolet radiation or heat is usually used. It has, however, been found that this method has a number of problems associated with display unevenness.
That is, as this method is a rubbing-less method, if there occurs even a slight change in the structure or in electric lines of force, the liquid crystal molecules may not align in the desired direction. As a result, there are cases where a contact hole or the like formed outside the display area disrupts the alignment of the liquid crystal molecules and the disruption affects the alignment of the liquid crystal molecules within the display area, resulting in the formation of an abnormal domain and causing the alignment to be held in that state. Furthermore, if structures that cause such disruptions in liquid crystal molecular alignment are located in the same alignment sub-region, abnormal domains formed from the respective structures are concatenated, forming a larger abnormal domain. This causes the liquid crystal molecules outside and inside the display area to be aligned in directions other than the desired directions, and the polymerizable component is polymerized in that state, resulting in such problems as reduced brightness, slower response speed, and display unevenness. FIG. 44 is a plan view showing a pixel of a comparative example. In the pixel shown here, contact holes that cause variations in cell thickness are not located at liquid crystal domain boundaries, and two contact holes are located within the same alignment sub-region. As a result, an abnormal domain is formed in such a manner as to connect the two contact holes and, with the alignment held in this state, the polymerizable component is polymerized, resulting in display performance degradations such as reduced brightness, slower response speed, and display unevenness.
Further, when a metal electrode such as a source electrode or a Cs intermediate electrode is extended into the display pixel, there occurs the problem of reduced numerical aperture, and hence, reduced brightness. Moreover, if an electrode with the same potential as the pixel electrode is extended into the display pixel, this also causes reduced brightness, slower response speed, and display unevenness.
In connection with a seventh aspect of the invention, while conducting studies on the technique in which a liquid crystal composition containing a polymerizable component is sandwiched between substrates and, while applying a voltage, the polymerizable component is polymerized, thereby defining the tilt direction of the liquid crystal molecules, the inventor et al. encountered the problem that when the same pattern was displayed for a certain length of time, image sticking occurred in the portion where the pattern was displayed. This is believed to occur because the polymerization is insufficient and the polymer deforms. On the other hand, to sufficiently polymerize the polymer, the duration of light radiation or heating must be increased, but in that case, tact time at the time of volume production becomes a problem.